1. Field of the Invention
The present invention relates to a matrix-type super twisted nematic (STN) liquid crystal display device used for various office automation (OA) equipment such as a personal computer and/or a word processor, a multi-media information terminal, audio video (AV) equipment, game equipment, etc., and a method for driving the same. More specifically, the present invention relates to a liquid crystal display device in which an improved uniform display quality can be obtained, and a method for driving the same.
2. Description of the Related Art
In the above-mentioned STN liquid crystal display device, higher response is known to decrease contrast. Hereinafter, the reason for this and proposed techniques for solving this problem will be described.
In the past, in an STN liquid crystal display device, a line sequential driving system has been adopted. According to this driving system, row electrodes are successively scanned one at a time over one frame period, high scanning pulse is applied to each row electrode only once in one frame period, and in synchronization with this, a data voltage in accordance with display data of each pixel on a row electrode to be scanned is applied to column electrodes.
In a liquidcrystal display device adopting the above-mentioned driving system, a display of mainly a freeze-frame picture is intended. Therefore, such a liquid crystal display device uses liquid crystal with relatively low response. In this case, liquid crystal responds to an effective value of a voltage to be applied, and a practical contrast ratio is obtained. However, when it is attempted to realize high response of the liquid crystal by reducing the viscosity of the liquid crystal and making a liquid crystal layer thinner so as to create a display of a moving picture, according to the line sequential driving system, the liquid crystal responds to a driving waveform itself, without responding to an effective value of a voltage, and the transmittance noticeably fluctuates per frame. This phenomenon is called a frame response phenomenon, which causes a significant decrease in the contrast ratio.
In order to solve the above-mentioned problem, the following driving system is proposed. According to this system, unlike the linear sequential driving system, a plurality of low scanning pulses are applied in one frame, thereby suppressing a frame response phenomenon to prevent the contrast ratio from decreasing. Such a driving system is called a multi-line simultaneous selection driving system, which is characterized in that a plurality of row electrodes are simultaneously scanned by using an orthogonal matrix. Hereinafter, its basic operation will be briefly described.
Input data is subjected to orthogonal transformation by using an orthogonal matrix, and a data voltage based on operational data is applied to a column electrode. In synchronization with this, a scanning voltage based on a column vector of the orthogonal matrix is simultaneously applied to row electrodes to be simultaneously selected. In this way, orthogonal transformation of image data is performed on a liquid crystal panel, and an input image can be reproduced. At this time, depending upon the number of row electrodes be simultaneously selected and the scanning order, the following three driving systems are proposed. The basic principle of each driving system is as described above.
The first driving system is an active addressing system in which all the row electrodes in one screen are simultaneously scanned. This system is disclosed in T. J. Scheffer, et al., SID ""92, Digest, p. 228, and Publication for Opposition No. 7-120147.
The second driving system is a sequence addressing system in which a plurality of row electrodes less than all the electrodes in one screen are classified into groups, and each group is sequentially scanned. This driving system enables the circuit size to be smaller, compared with the first driving system. This system is disclosed in T. N. Ruckmongathan et al., Japan Display ""92, Digest, p. 65 and Japanese Laid-Open Publication No. 5-46127.
According to the third driving system, one screen is divided into a plurality of blocks in a row direction, and a plurality of row electrodes less than all the electrodes in each block are classified into groups, and each group is sequentially scanned, whereby all the blocks driven (Japanese Laid-Open Publication No. 6-291848). This driving system can reduce memory space, and enables the circuit size to be smaller, compared with the second driving system.
As described above, by adopting a multi-line simultaneous selection driving system in a high-response simple matrix-type liquid crystal display device, the frame response phenomenon is suppressed, and a decrease in the contrast ratio can be prevented.
Furthermore, as a gray-scale system of an STN liquid crystal display device, a pulse width modulation system (PWM system) is known, in which an operation between display data and an orthogonal function is performed per bit, and a data signal voltage with a width corresponding to a weight per bit is applied to a column electrode. This is disclosed in Japanese Laid-Open Publication No. 990914.
However, in the conventional pulse width modulation system, the number of signal changes per horizontal scanning period becomes larger than the case where a gray-scale display is not realized. This increases the frequency of a data voltage signal, so that the amount of induced distortion caused by electrode resistance and liquid crystal capacitance becomes large. As a result, an effective voltage value slightly different from the fundamental effective voltage value is applied to liquid crystal, which leads to a decrease in display quality due to, for example, cross-talk. This will be described with reference to the drawings.
As shown in FIG. 8, a display state of pixels is determined on a liquid crystal panel including 6 pixels in the vertical (row) direction and 2 pixels in the horizontal (column) direction. Each of column electrodes X1 and X2 and row electrodes Y1 to Y6 is determined as shown in this figure. FIG. 9 shows driving waveforms C1C and C2C of the column electrodes X1 and X2 and a driving waveform R1C of the row electrode Y1 in the case where the liquid crystal panel in this state is driven in the conventional manner.
As is understood from FIG. 9, according to the conventional driving system, display data on the column electrode X1 is exactly the same as that on the column electrode X2, so that a data signal represented by the waveform C1C applied to the column electrode X1 becomes exactly the same as a data signal represented by the waveform C2C applied to the column electrode X2. Thus, timing of changes in waveforms and the change directions are exactly the same between the two signals. Therefore, waveform distortion of a scanning voltage represented by the waveform R1C induced by a data voltage becomes relatively large as shown in FIG. 9. Accordingly, a waveform of a voltage actually applied to liquid crystal is largely shifted from an ideal waveform without containing any waveform distortion, and an effective voltage value becomes substantially different from an ideal value.
Thus, considering the case where hundreds of column electrodes are provided, the variation in difference between the effective voltage value and the ideal value becomes large per column electrode according to the conventional driving method, which leads to a decrease in display quality due to, for example, cross-talk.
A method for driving a liquid crystal display device is provided. The device includes a plurality of row electrodes to which a scanning signal is applied; a plurality of column electrodes disposed so as to cross the plurality of row electrodes, to which a display data signal is applied; and a liquid crystal layer interposed between one of the plurality of row electrodes and one of the plurality of column electrodes, for performing a display at a crossing portion of one of the plurality of row electrodes and one of the plurality of column electrodes in response to an effective value of a voltage applied across one of the plurality of row electrodes and one of the plurality of column electrodes, wherein gray-scale display data consisting of a plurality of bits is received, and a voltage of a display data signal selected per bit by a scanning signal is applied during a period in which weights are assigned to bits during one horizontal scanning period and a voltage applying timing is made different at at least one column electrode, whereby a gray-scale display is realized.
In one embodiment of the present invention, timing of applying a voltage of a display data signal during a period in which weights are assigned to bits during the one horizontal scanning period, and a voltage applying timing is made different at at least one column electrode is adjusted, and a direction of a waveform change of a first display data signal applied to at least one first column electrode at a first timing is opposite to a direction of a waveform change of a second display data signal applied to at least one second column electrode to which a display data signal is applied at a second timing.
In another embodiment of the present invention, timing of applying a voltage of a display data signal during a period in which weights are assigned to bits in the one horizontal scanning period, and timing at which a voltage is applied is made different at at least one column electrode is adjusted, in such a manner that the voltage applying periods of adjacent horizontal scanning periods are connected.
In another embodiment of the present invention, timing of applying a voltage of a display data signal during a period in which weights are assigned to bits in the one horizontal scanning period, and timing at which a voltage is applied is made different at at least one column electrode is adjusted, in such a manner that the voltage applying periods of adjacent horizontal scanning periods are connected.
A liquid crystal display device of the present invention includes a plurality of row electrodes to which a scanning signal is applied; a plurality of column electrodes disposed so as to cross the plurality of row electrodes, to which a display data signal is applied; and a liquid crystal layer interposed between one of the plurality of row electrodes and one of the plurality of column electrodes, for performing a display at a crossing portion of one of the plurality of row electrodes and one of the plurality of column electrodes in response to an effective value of a voltage applied across one of the plurality of row electrodes and one of the plurality of column electrodes. The device further includes: a pulse control portion for receiving gray-scale display data consisting of a plurality of bits, and determining timing at which a voltage of a display data signal selected per bit by a scanning signal by assigning weights to bits in one horizontal scanning period; and a column driver for applying the voltage of the display data signal to at least one column electrode, based on the timing selected by the pulse width control portion.
Hereinafter, the function of the present invention will be described.
According to the present invention, gray-scale display data consisting of a plurality of bits is received, and a display data signal selected per bit by a scanning signal is applied only during a period in which weights are assigned to bits in one horizontal scanning period, and the timing of applying a voltage is made different at each of the column electrodes or at at least one of the column electrodes. Therefore, as represented by (b) in FIG. 10, even under the condition of the same gray-scale display data and the same scanning signal, waveforms C1K and C2K of display data signals on different column electrodes change at different times during the same horizontal scanning period. Thus, a degree (amplitude) of distortion of a waveform R1K of a scanning signal induced by the display data signals can be suppressed.
Furthermore, timing of applying a display data signal only during a period in which weights are assigned to bits during one horizontal scanning period and timing of applying a signal is made different per column electrode is adjusted. As represented by (c) in FIG. 10, one column electrode to which a display data signal C and another column electrode to which a display data signal C2L is applied are set so as to have opposite directions of waveform change. Thus, when a pulse of one column electrode falls, a pulse of another column electrode rises. Since timing of waveform change and direction of change are different between these two column electrodes, waveform distortion of a scanning signal induced by a data display signal is relatively small and dispersed in terms of time. As a result, a shift of a waveform of a voltage actually applied to liquid crystal from an ideal waveform without any waveform distortion becomes relatively small, and the difference between the effective voltage value and the ideal value does not become so large.
As represented by (c) in FIG. 10, timing of applying a display data signal is made opposite between two column electrodes. However, the present invention is not limited thereto. For example, directions of waveform change may be made opposite between at least two column electrodes to which a display data signal is applied at an identical timing and at least two column electrodes at which a display data signal is applied at another identical timing. Furthermore, the present invention is similarly applicable between one column electrode and a plurality of column electrodes. Furthermore, column electrodes to which a display data signal is applied at an identical timing or at another identical timing may be adjacent to each other or separately disposed. Furthermore, even in the case where a plurality of electrodes are supplied with a display data signal at an identical timing and a plurality of electrodes are supplied with a display data signal at another identical timing, respective column electrodes in each column electrode group may be separately disposed.
Furthermore, when the timing of applying a display data signal only during a period in which weights are assigned to bits during one horizontal scanning period, and-the timing of applying a signal is made different per column electrode is adjusted in such a manner that the signal applying periods of adjacent horizontal scanning periods are connected (see (d) in FIG. 10), a cycle of waveform change becomes long. Therefore, the number of times waveform distortion of a scanning signal is induced by a data display signal is reduced. As a result, a shift of a waveform of a voltage actually applied to liquid crystal from an ideal waveform without any waveform distortion becomes relatively small, and the difference between the effective voltage value and the ideal value does not become so large. Furthermore, according to this method, a waveform blunting of a data display signal itself is reduced, and a voltage of a data display signal becomes closer to an ideal value, compared with the conventional method.
Furthermore, by combining the relationship represented by (c) in FIG. 10 and the relationship represented by (d) in FIG. 10, as represented by (e) in FIG. 10, the number of times waveform distortion of a scanning signal occurs can be reduced, compared with the case of using the relationship represented by (b) in FIG. 10.
Thus, the invention described herein makes possible the advantage of (1) providing a liquid crystal display device in which cross-talk can be decreased; and (2) proving a method for driving the liquid crystal display device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.